Typical Constructions of Overhead Lines _ EEP

5/27/2015 Typicalconstructionsofoverheadlines|EEP

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TypicalConstructionsOfOverheadLines

FIGURE 2.1 - Example overheaddistribution structures. (a) Three-phase34.5-kV armless construction with covered



wire.

Along streets, alleys, through woods, and in backyards, many of the distribution lines that feedcustomers are overhead structures.

Because overhead lines are exposed to trees and animals, to wind and lightning, and to cars andkites, they are a critical component in the reliability of distribution circuits. Overhead constructionscome in a variety of configurations (see Figure 2.1).

Normally one primary circuit is used per pole, but utilities sometimes run more than one circuit perstructure. For a three-phase circuit, the most common structure is a horizontal layout with an 8- or10-ft wood crossarm on a pole (see Figure 2.2). Armless constructions are also widely found wherefiberglass insulator standoffs or post insulators are used in a tighter configuration.

Utilities normally use 30- to 45-ft poles, set 6 to 8 ft deep. Vertical construction is also occasionallyused. Span lengths vary from 100 to 150 ft in suburban areas to as much as 300 or 400 ft in ruralareas.

Distribution circuits normally have an underbuilt neutral the neutral acts as a safety ground forequipment and provides a return path for unbalanced loads and for line-to-ground faults. Theneutral is 3 to 5 ft below the phase conductors.

Utilities in very high lightning areas may run the neutral wire above the phase conductors to act as ashield wire.

Some utilities also run the neutral on the crossarm. Secondary circuits are often run under theprimary. The primary and the secondary may share the neutral, or they may each have their ownneutral. Many electric utilities share their space with other utilities telephone or cable televisioncables may run under the electric secondary.

FIGURE 2.1 Continued. (b)



Single-phase circuit, 7.2 kVline-to-ground

FIGURE 2.1 Continued. (c) Single-phase, 4.8-kV circuit

FIGURE 2.1 Continued. (d) 13.2-kV



spacer cable

Wood is the main pole material, although steel, concrete, and fiberglass are also used. Treated woodlasts a long time, is easy to climb and attach equipment to, and also augments the insulation betweenthe energized conductors and ground.

Conductors are primarily aluminum. Insulators are pin type, post type, or suspension, eitherporcelain or polymer.

FIGURE 2.2 - Example crossarm construction.

The National Electrical Safety Code (IEEE C2-2000) governs many of the safety issues that playimportant roles in overhead design issues. Poles must have space for crews to climb them and worksafely in the air. All equipment must have sufficient strength to stand up to normal operations.Conductors must carry their weight, the weight of any accumulated ice, plus withstand the windpressure exerted on the wire. We are not going to discuss mechanical and structural issues in thisbook.

Overhead construction can cost $10,000/mi to $250,000/mi, depending on the circumstances. Someof the major variables are labor costs, how developed the land is, natural objects (including rocks inthe ground and trees in the way), whether the circuit is single or three phase, and how big theconductors are.

Suburban three-phase mains are typically about $60,000 to $150,000/mi single-phase laterals areoften in the $40,000 to $75,000/mi range. Construction is normally less expensive in rural areas inurban areas, crews must deal with traffic and set poles in concrete. As Willis (1997) notes, upgradinga circuit normally costs more than building a new line.



Typically this work is done live: the old conductor has to be moved to standoff brackets while thenew conductor is strung, and the poles may have to be reinforced to handle heavier conductors.

SOURCE: El. Power Distribution Equipment and Systems A. Short

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highlyspecialized for design of LV high power busbar trunking (

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Typical Constructions of Overhead Lines _ EEP